Pigments are dispersed as fine particles into a liquid medium for use in coatings, inks, molten plastics, cosmetics and adhesives. It is desirable for the pigment to be dispersed as finely as possible and as rapidly as possible into the liquid medium and remain as a stable fine dispersion over time for optimum results. Unfortunately, the dispersion of fine particles in liquids is unstable in that the particles tend to agglomerate or flocculate causing uneven pigmentation in the product where pigmentation is desired. To minimize the effects of agglomeration or flocculation, surface active pigment dispersants have been developed.
It is known that polymers with amine functional groups or polymers with heterocyclic bases or their salts are effective in adhering to the surfaces of the pigment particles. The adhesion of the dispersant to the particle surfaces of the pigment prevents agglomeration or flocculation and minimizes the time and force needed to disperse the pigment into a medium that is suitable for the end use. The amine functional polymers have been found to be particularly effective for pigments which are difficult to disperse, such as high color carbon black and organic pigments. Amine functional polymers produced by the reaction of polyethyleneimine and a polyester are described in U.S. Pat. Nos. 5,700,395, 4,645,611, 4,415,705 and 4,224,212 as pigment dispersants. Pigment dispersants wherein basic heterocycle groups with polyisocyanates are attached to polyesters and polyethers are taught in U.S. Pat. No. 4,942,213.
However, there are a number of disadvantages with such dispersants. The amine functional groups in these dispersants neutralize the acid catalysts used in acid catalyzed processes, e.g., the melamine cross linking process. For this reason the catalyst demand is increased or a higher cure temperature is required. The amine functional dispersants also cause problems in isocyanate crosslinking processes because they react with isocyanate and catalyze the isocyanate/H2O reaction. Both the melamine crosslinking and the isocyanate crosslinking processes are widely used in coatings, inks, and adhesives. The undesirable amine reaction with isocyanate increases the viscosity of the coating or printing ink formulations causing problems in application. As a result more solvent is required to lower the viscosity. This leads to an undesirable increase in VOC (volatile organic compounds). Moreover, the catalysis of the isocyanate/H2O reaction leads to increased gassing, which causes more surface defects in two component polyurethane coating systems. Further, the amine functional polymeric dispersants tend to yellow or discolor when exposed to UV light. The amine functional dispersants are potentially cationic polymers, which are suspected to be highly toxic toward aquatic organisms (“Polymer Exemption Guidance Manual” from EPA, May 22, 1997, Page 8). Therefore, it would be desirable to have non-amine functional catalysts with the good dispersant characteristics of amine functional dispersants without the problems caused by the presence of amine functional groups.
To solve these problems, dispersants with functional groups other than amines have been developed. For example, dispersants with acid functional groups prepared from acrylic acid polymer, styrene maleic anhydride copolymer and alkene maleic anhydride copolymer are known. U.S. Pat. No. 4,673,705 and U.S. Pat. No. 4,754,056 disclose radiation curable dispersants that are acrylate-functional carboxylic acid polyamides carrying free carboxyl groups. The polyamide is an adduct of a polyacrylate with a stoichiometrically deficient amount of a primary amine. Any secondary amine groups that may be present are also converted to an amide with a carboxylic acid anhydride. U.S. Pat. No. 6,037,414 disclosed a dispersant, a polyacrylate chain with cyclic imide functional groups and polyester side chains. U.S. Pat. No. 5,151,218 described phospohoric acid esters as suitable dispersants. The phosphoric acid ester is prepared by reacting phosphoric acid or a phosphate forming compound with a hydroxy compound that contains ether groups and ester groups or urethane groups. An example of a suitable polyether-polyester is the reaction product of a monofunctional polyether with caprolactone. A radiation curable pigment composition wherein the dispersant is an adduct of a mono functional polyether amine and maleic anhydride is described in U.S. Pat. No. 4,496,686. However, these dispersants generally are not as effective as amine functional dispersants in dispersing high color carbon black or organic pigments.
It is desirable to have a pigment dispersant that is as effective as the known amine functional dispersants but without amine functional groups or with a reduced amine content so that the dispersant can be used in acid catalyzed or isocyanate processes without the adverse effects of amine functional dispersants. It is also desirable to have an effective pigment dispersant with less toxicity than the known amine functional dispersants.